The present invention relates to a method and apparatus for end-to-end joining of paper roll cores. Conventionally, a paper web is wound up during different finishing operations on a core made from a plurality of spirally overlappingly wound plies of narrow strips of board. Prior to their winding into a core, the board strips are glued, conventionally using a sodium silicate solution. The finished core length must be exactly matched with the width of the paper web exiting from a slitter and being wound on the core. Furthermore, the core must be flawless to avoid problems with the chucks of winder equipment employed in the final use and/or finishing of the roll.
A used core presents an essential waste problem, since its material as such is not reusable. Consequently, while substantial effort has been made to find possibilities of recycling cores, this approach also involves problems. One problem is the damage caused to the core ends during normal use. Conventionally, methods of overcoming this drawback have been sought from reworking of core sections. A basic goal of reworking is to remove the damaged portion from the core end and then to join the thus reworked core end with the end of another core section similarly reworked so as to form a continuous core master that can be severed to desired lengths
In the art, different kinds of methods and apparatuses have been developed for reworking and joining core ends. One type of apparatus for smoothing and end-to-end joining of core ends is disclosed in SE patent publication No. 502,067. Respectively, the SE laid-open publication 470,442 describes a method and apparatus suited for working the ends of cores to be reworked into complementary mating outer/inner end cones that are then joined with glue.
In the method described in the latter publication, the core ends that are first trimmed straight are reworked using conical milling equipment. The mill working the core end into an inner cone is provided with a milling head dimensioned according to the diameter of the core being reworked. The milling head is aligned coaxially with the longitudinal axis of the core. Respectively, the outer cone end is worked using conical milling equipment performing a rotary movement about an axis perpendicular to said longitudinal axis of the core, whereby the outer edge of the core end will be worked into a tapering cone having a cone angle determined by the envelope angle of the conical cutting surface of the milling head, as the core end is rotated past the milling head. In cited embodiment, the cores being reworked are rotated on two support rolls mounted parallel to the longitudinal axis of the core by means of a friction drive roll which is placed above the core so as to run on the surface thereof.
An essential drawback of the above-described method and the apparatus implementing the method is that the possible out-of-roundness of the core cross section cannot be corrected by any means during the reworking of the core to be recycled. In fact, the working tools of a fixed shape and aligned to the estimated center axis of the core perform the reworking of the core ends in rigid manner irrespective of any possible out-of-roundness deviations of the core cross section. Obviously, this causes unavoidable mismatch problems in joining the core ends when a core with an out-of-round end is to be reworked. Moreover, cited apparatus is handicapped by having the reworking of core ends and the end-to-end joining thereof arranged to occur in separate machine units.
The present invention provides a method and an apparatus for joining the ends of cores such that the cores are reworked using a substantially reduced number of steps as compared to those required in the prior-art techniques. Moreover the present method outperforms the prior an methods by making a more accurate end-to-end joint between the core ends, as well as a straighter joined core master with a cross section of good roundness.